The present invention generally relates to processing of material in a manufacturing plant and, more particularly, to methods and systems for tracking the movement of wafers in a semiconductor processing plant.
Conventional manufacturing plants move material to be processed through a manufacturing process having several processing areas. Currently these material lots are tracked in larger quantities that may be disposed in a carrier for ease of movement throughout the facility.
Some manufacturing processes require that the item being processed be rotated regularly in order to ensure that the item is properly processed, such as when painting an object or when applying a coating to a substrate. In the case of a mechanical process, the object is rotated to ensure that the tooling is being worn evenly or that the tooling is mechanically treating the object evenly. Even though some of these items may be individually processed, or processed in small lots, the items may form part of a larger lot being manufactured and it is difficult to distinguish the progress of the individual item as it moves through the processing line. As the number of processing steps increase tracking becomes even more difficult. This is particularly a problem in the processing of wafers in a semiconductor processing plant.
A conventional semiconductor fabrication plant typically includes multiple fabrication areas or bays interconnected by a path, such as a conveyor belt. Each bay generally includes the requisite fabrication tools (interconnected by a subpath) to process semiconductor wafers for a particular purpose, such as photolithography, chemical-mechanical polishing or chemical vapor deposition, for example. Material stockers or stocking tools generally lie about the plant and store semiconductor wafers waiting to be processed. Each material stocker typically services two or more bays and can hold hundreds of cassettes. The wafers are usually stored in cassettes in groups of about 25 wafers. The wafers are then disposed within a carrier and move from one process to another in the carrier. The carriers are usually tracked by their carrier code by a computer system as they move through the plant.
Once a lot has been retrieved, and the equipment has been set up, the operation on the wafers by a particular piece of equipment, or xe2x80x9ctool,xe2x80x9d can begin. At this point, the lot is xe2x80x9cmoved-inxe2x80x9d to the operation. An operator on the line then communicates this information to the host computer. The lot remains in this state until the operation is completed. Once the operation is completed, the operator must perform tests and verifications on the wafers. When all tests and verifications have been performed, the host computer application program must be notified. Wafers may have moved from one cassette to another as a result of the operation; therefore the host application and computer has to be notified of these moves. The operator then places the cassette of xe2x80x9cmoved-outxe2x80x9d wafers in the material stocker to await orders as to the location of the next piece of equipment that will perform operations on the wafers.
The semiconductor fabrication plant, including the bays, material stockers and the interconnecting path, typically operates under control of a distributed computer system running a factory management program. In this environment, the automated material handling system (AMHS) may conceptually include the cassettes, the transportation system (e.g., paths) and control system (e.g., the distributed computer system). An empty carriers management system as well as a separate test wafer management system may also form part of the AMHS.
Data gathered during the course of wafer processing is used to diagnose yield problems and forms the basis of yield improvement efforts. Such data includes parametric electrical test data gathered on individual circuits and test structures fabricated on the wafers, as well as wafer sort data which tests the suitability for use of the wafers once wafer processing is completed. One of the possible sources of yield variation is the order in which wafers in a lot are processed at a given processing step. When the processing is done one wafer at a time per step, a variation in yield may be caused by a build up of contaminants, uneven heating of a processing chamber or another physical aspect that changes during the processing of the lot. In a batch operation, the physical location of the wafer in the batch processing equipment may influence uniformity of the processing effects across the lot. In an example where wafers are moving through a contaminated chamber, if the order in which each wafer is processed is known then the final wafer yield may be plotted against the processing order in this step. For each wafer in a lot a drop-off in yield versus processing order would be observed due to the contamination problem. This data is used to make adjustments to the line to improve yield; however, this wafer tracking method lacks the level of precision in the data collected required by chip plants today.
In tracking the wafer processing order, specialized equipment has been used to read scribed wafer identifiers, either immediately prior to or after critical processing steps, and to store this data for later correlation with device performance. Randomizing the order of the wafers prior to such steps is often done to ensure effects are not compounded. The wafer positional data is fed into a computer system, the device performance metrics for a wafer lot of interest are manually entered, and then all possible graphs of the device metrics for that lot versus wafer processing order at each step are generated. The data is then reviewed to determine those steps at which the processing order may affect performance. This type of approach to tracking wafers can be costly in its implementation due to the amount of hardware and software needed to randomize the wafer order and interface with the wafer processing system""s main computer database.
The present invention is directed to addressing the above and other needs in connection with improving traceability and yield of wafers as they move through a multiple step process.
In the case of multiple stage wafer processing, once the wafer is presented at the start of the processing line the opportunity to track the individual wafer and the wafer""s progress is no longer available. In view of the above, there is a need to implement a wafer tracking method that has a high level of precision as an individual wafer is moving within a multiple stage processing line. There is also a need to implement a wafer tracking system that provides a feedback component for making adjustments on the processing parameters.
According to one aspect of the invention, it has been discovered that wafer tracking in a multiple stage processing line can be improved by rotating a wafer and recording the rotation angle and the wafer""s location in the processing line each time the wafer moves. Each bit of data collected represents a set of coordinates that is used to develop a wafer movement map for correlating with other wafer movement maps to identify processing locations or tooling that are causing deviations to occur on the wafer surface. It has also been discovered that the yield of successively processed wafers is improved by integrating into the processing system an in-situ feedback component for adjusting processing parameters of earlier processing stages.
According to another aspect of the invention, a method for detecting a processing deviation in a multiple stage wafer processing system, having at least two processing parameters, includes determining an angle of rotation of a wafer as the wafer is presented to various stages of processing. The rotation angle and the wafer""s corresponding location are recorded each time the wafer moves to another stage in the processing line. After the wafer is processed the wafer is analyzed for any surface deviations. Any wafer surface deviations are then correlated to at least one of the processing parameters and to at least one processing location within the wafer processing system.
According to another aspect of the invention, a system detects a processing deviation in a multiple stage wafer fabrication process having at least two processing parameters. The system includes a scanning device that determines an incoming angle of rotation of a wafer as it moves into a first processing stage and determines a translation angle as the wafer is within the first stage. A rotating device for rotating the wafer to an outgoing angle of rotation as the wafer is exiting the first stage and moving into a second processing stage. A computer arrangement is included that records each angle of rotation and a corresponding processing location of the wafer as the wafer moves through each stage of wafer processing. An analyzer arrangement detects deviations in the processed wafer while the computer arrangement performs a correlation analysis of the wafer data to correlate the wafer deviations to at least one of the processing parameters and processing location within the processing system.
In yet another aspect of the invention, a method for detecting a processing deviation in a multiple stage wafer processing system, having at least two processing parameters, includes determining an angle of rotation on a wafer as it is presented to various processing stages. A processing recipe is applied to the wafer as it is moving through the various processing stages. The angle of rotation of the wafer and a corresponding processing location of the wafer is recorded as the wafer moves through each stage of the wafer processing system. Mid-way through, or at the end of, wafer processing the wafer is analyzed for any deviations in relation to the processing recipe to determine the existence of a processing deviation from the recipe. The data gathered from the wafer deviation analysis is then correlated to at least one of the processing parameters and at least one of the processing locations within the wafer processing system